/Engine/Source/Runtime/Engine/Classes/Engine/SkinnedAsset.h
Take the BoneSpaceTransforms array (translation vector, rotation quaternion and scale vector) and update the array of component-space bone transformation matrices (ComponentSpaceTransforms). It will work down hierarchy multiplying the component-space transform of the parent by the relative transform of the child. This code also applies any per-bone rotators etc. as part of the composition process
void FillComponentSpaceTransforms ( const [TArray](API\Runtime\Core\Containers\TArray)< FTransform > & InBoneSpaceTransforms, const [TArray](API\Runtime\Core\Containers\TArray)< FBoneIndexType > & InFillComponentSpaceTransformsRequiredBones, [TArray](API\Runtime\Core\Containers\TArray)< FTransform > & OutComponentSpaceTransforms ) const